The reduction in size of electronic equipment has been accompanied by an increase in the use of liquid crystal displays (LCDs). The LCD is not only used as a computer screen, but also is used as a television screen, a projection screen, etc. Utilizing liquid crystal has advantages such as low power consumption due to low driving voltage, and relatively fast response. It is expected that the field of application of LCDs will expand in the future.
Most of the currently used LCDs are of the active matrix type. The active matrix type means the one in which a separate driving circuit element is provided for each pixel to improve display characteristics. Active matrix LCDs using thin-film three-terminal transistors (TFTs) as switching elements are called TFT liquid crystal displays (TFTLCDs).
In using TFTLCDs to display pictures, it is necessary to provide gray scale data of the picture to the LCD to drive the LCD. FIG. 1 shows the construction of the control unit of the TFTLCD. The array/cell portion 1 of the LCD is connected to an X-driver 3 and a Y-driver 5. The X-driver 3, when it is supplied with gray scale data, applies a voltage corresponding to the gray scale data to the cell. The Y-driver 5 is connected to the gate of a switching element, and conducts/does not conduct the voltage applied to the cell by the X-driver 3 at a predetermined time.
Gray scale data is supplied to the X-driver by data control unit 10. The data control unit 10 consists of a data control circuit 12 for latching and storing the externally supplied R/G/B data in a buffer, and a timing control circuit 14 for outputting the gray scale data stored in the buffer to the X-driver 3 at a predetermined time. A clock signal is externally supplied to the data control circuit 12 and the timing control circuit 14 to control the timing. A power supply 7 is connected to the X-driver, Y-driver 5, and data control unit 10.
To display a picture on an LCD, a voltage corresponding to the gray scale is provided to each pixel of each color. That is, the driving of a pixel is not a simple on-off function, a voltage divided into several levels (gray scales) is provided to adjust the transmissivity of the pixel, so that intermediate color intensity can be displayed. To achieve such control in a color display, R/G/B signal levels are supplied to each pixel. For a display of a 64-level gray scale, 64-step voltage is used, and the voltage for each pixel is applied according to the respective gray scale data. Ideally, the same transmissivity can be achieved for all the colors when the voltage corresponding to a particular gray scale is used. The relationship for this is shown in FIG. 2. In FIG. 2, transmissivity is plotted on the ordinate, and applied voltage is plotted on the abscissa. Applied voltage is determined by the gray scale data. Accordingly, when a certain gray scale n is chosen, the applied voltage Vn is determined by that gray scale. Then, according to the relationship of FIG. 2, transmissivity Tn for the gray scale Vn is achieved.
Ideally, the relationship between gray scale, applied voltage, and transmissivity is the same for each of the R/G/B colors. However in actuality, the gray scale and the achieved transmissivity have a slight difference depending on color. This is because the degree of light modulation for the specific twist of the twisted noematic liquid crystal is slightly different depending on wavelength. That is, even though a light passes through a liquid crystal layer in a similarly twisted state, the degree of the modulation given to the passing light is wavelength dependent, and thus the scattering of brightness that occurs for a given gray scale is color dependent. This is shown in FIG. 3. The transmissivity of blue (B) is higher than that of both red (R) and green (G) for the same voltage over a wide range of applied voltage. That is, since the relationship between gray scale and applied voltage for each color is unique, the transmissivity of blue (B) is greater even if each color is selected with the same gray scale and the same voltage is applied in the displaying of intermediate colors. Thus, the correlation between transmissivity and applied voltage (hereinafter referred to as transmissivity/applied voltage characteristics) has a color (wavelength) dependency. If the displaying is performed without providing any correction, the graduation of color translates to blue more than called for by the halftone data, and the picture on the whole takes on a bluish hue. FIG. 4 shows this state represented by a chromaticity diagram. FIG. 4 shows that L63 should be a white color state if an ideal state could be realized, but in actuality, L0, or a shift to blue, occurs because of the wavelength dependency of the transmissivity/applied voltage characteristics.
Various methods have been proposed for correcting the above problem. These methods are roughly divided into (1) methods for making the correction by the modification of the structure of LCD, and (2) methods for making the correction by using electric control.
A typical example of the first category (1) is the adoption of a multi-gap structure. A multi-gap structure is a structure in which the thickness of the color filter of the pixel of each color of R/G/B varies. That is, the thickness (gap) of the liquid crystal sealing portion is changed to achieve the matching of the transmissivity/applied voltage characteristics for each color. However, implementation of a multi-gap structure is accompanied by difficulties in the manufacturing process. Problems occur in the adjustment of the thickness of the color filter, and in the uniformization of the gap between the two glass substrates forming the liquid crystal cell. Yield is effected by these difficulties causing an increase in manufacturing cost.
As an example of the second category (2), is a method in which the reference voltage (gray scale voltage) given to the data driver is tailored to the characteristics for each color. This method can compensate for the color dependency of the transmissivity/applied voltage characteristics. However, the circuits needed to independently control the reference voltages, raise the cost and cause difficulties in the implementation. Another method that falls within this second category, is to use the voltage for one of the colors of R/G/B as a reference voltage, and use offset voltages for each of other colors. This methods has the same problems as the method in which the reference voltages are separately applied, and in addition, cannot accomplish desired effect if the gradients of the curves showing the transmissivity/applied voltage characteristics of R/G/B vary with applied voltage. That is, in accordance with the offset voltage method, correction is carried out by applying a uniform offset voltage for all applied voltages, and thus the correction cannot be effectively performed unless the gradients of the curves showing the transmissivity/applied voltage characteristics are the same over the whole applied voltage range.
Japanese Published Unexamined Patent Application No. 01-101586 discloses a technique in which different liquid crystal driving voltage levels are set for each of the colors, and that level is applied to each pixel. Japanese Published Unexamined Patent Application No. 03-6986 discloses a technique in which the driving voltage is made to vary a predetermined voltage from color to color to obtain uniformity in transmissivity. Japanese Published Unexamined Patent Application No. 03-290618 discloses a technique in which a similar object is accomplished by independently inputting a gray scale control signal for each color.
Therefore, first object of the subject invention is to provide a driving method for a TFTLCD in which the dependency on color of the transmissivity/applied voltage characteristics is effectively corrected.
A second object of the subject invention is to realize the effective correction using a very simple method which enables the above described correction to be made without increase in complexity of the control method, and the restrictions on the implementation by addition of circuits.